Puppet’s special default value usually acts like a keyword in a few limited corners of the language. Less commonly, it can also be used as a value in other places.

Syntax

The only value in the default data type is the bare word default.

Usage

The special default value is used in a few places:

Cases and selectors

In case statements and selector expressions, you can use default as a case, where it causes special behavior. Puppet will only try to match a default case last, after it has tried to match against every other case.

All of the resources in the block above will inherit attributes from default unless they specifically override them.

Parameters of data types

Several data types take parameters that have default values. In some cases, like minimum and maximum sizes, the default value can be difficult or impossible to refer to using the available literal values in the Puppet language. For example, the default value of the String type’s max length parameter is infinity, which can’t be represented in the Puppet language.

These parameters often let you provide a value of default to say you want the otherwise-unwieldy default value.

Anywhere else

You can also use the value default anywhere you aren’t prohibited from using it. In these cases, it generally won’t have any special meaning.

There are a few reasons you might want to do this. The main one would be if you were writing a class or defined resource type and wanted to give users the option to specifically request a parameter’s default value. Some people have used undef to do this, but that’s tricky when dealing with parameters where undef would, itself, be a meaningful value. Others have used some gibberish value, like the string "UNSET", but this can be messy.

In other words, using default would let you distinguish between:

A chosen “real” value

A chosen value of undef

Explicitly declining to choose a value, represented by default

In other other words, default can be useful when you need a truly meaningless value.